hyperv: Fix vmops.get_info raises InstanceNotFound KeyError

Fixes a problem with the nova.virt.hyperv.vmops.get_info method raising
an InstanceNotFound with the wrong key for the exception message.  Also
adds a test case to make sure it works.

Fixes bug 1197506

Change-Id: I393951ff7bf6e28ce5d7a060cdd0011fe8ecc9ad
Merge "hyperv: Fix vmops.get_info raises InstanceNotFound KeyError"